Apparatus which uses a hygroscopic liquid such as a solution of lithium chloride, lithium bromide, ethylene glycol or the like to dehumidify air has been known for many years, and has found relatively wide application in certain special fields. Such apparatus, however, has been found to be comparatively inefficient from the standpoint of energy requirements when used in connection with air conditioning systems designed primarily to maintain comfort conditions for human occupants of buildings. The principal problem involves the necessity for expanding substantial amounts of energy to regenerate the hygroscopic liquid and the return to the system as a load, of a significant percentage of the energy required for regeneration. Apparatus which uses a solid desiccant such as silica gel, solid lithium chloride, activated alumina or the like similarly involved the necessity for expending substantial amounts of thermal energy for regeneration.